Method for making a fluid cooled acyclic generator rotor

ABSTRACT

An internally cooled rotor for an acyclic generator is disclosed as having a ferromagnetic steel core to which is diffusion bonded a cylinder copper conductor in which is embedded a multiplicity of cooling tubes communicating with coolant passages formed in the core. The cooling tubes are implanted in a hot isostatic pressure process during which the copper cylinder is at least in part created by the densification of copper powder to a non-porous mass.

The present invention relates to electromagnetic machines andparticularly to fluid cooled rotors for acyclic or homopolar generators.

Heretofore, acyclic generators, also known as homopolar or unipolargenerators, have generally been designed as low power, constant duty DCgenerators or short duty cycle pulse generators. In these designs, thelosses in the rotor are relatively low to the point that generatorperformance has not been unduly limited. To increase the powercapabilities and duty cycle of such DC generators without significantlyincreasing their physical size, the power density therein must beincreased. This can be achieved by driving the rotor to higherrotational and thus circumferential velocities. The higher velocitiesreduces the flux magnitude required to produce a given voltage and thusthe volume of material necessary to carry the main flux. Since acyclicgenerators are by their nature high DC current machines, and since theirrotors must carry current as well as magnetic flux, the rotor currentdensity becomes quite high as power generating capacity is increased.

The rotor of a typical iron core acyclic generator consists of aferromagnetic steel core for carrying the main flux with one or morecopper conductors affixed to the core periphery for carrying the highcurrent. These conductors may be in the form of a copper cylinder orsleeve shrunk-fit on or adhesively bonded to the core periphery.Resorting to copper conductors, rather than the core steel, as thepredominant current carrier, obviously reduces losses associated withhigh current densities. In addition, the presence of the copperconductor material reduces the effective rotor inductance and thus themagnitude of circumferential flux in the core steel. These factorsminimize the field coil magnetomotive force and improve the transientcurrent response of the rotor.

As the rotor current density is increased, dissipating the heatgenerated in the rotor current conductor becomes a major limitingfactor. If the rotor is not adequately cooled, the mechanical stressesresulting from centrifugal forces and differential thermal expansioncould destroy the rotor conductor structural integrity, since steel andcopper alloy tensile strength decrease with increasing temperature.

It is accordingly an object of the present invention to provide animproved rotor for an acyclic generator.

A further object is to provide an acyclic generator rotor of theabove-character which is capable of accommodating extremely highgenerator power densities.

Another object of the present invention is to provide an acyclicgenerator rotor of the above-character having provisions to accommodateinternal cooling of the current conductor.

An additional object is to provide an acyclic generator rotorconstructed with internal passages to accommodate the flow of a fluidcoolant effective in removing heat from the current conductor.

Yet another object of the present invention is to provide a method ofmanufacturing an internally cooled acyclic generator rotor of theabove-character.

Other objects of the invention will in part be obvious and in partappear hereinafter.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a rotor fora high power density acyclic generator having a one-piece ferromagneticsteel core for carrying the main flux and a high conductivity cylinder,preferably of copper, for carrying the main current; the latter beingdiffusion bonded to the core periphery. The core is provided withaxially extending passageways, which communicate with radially extendingpassages and axially extending conduits embedded in the copper cylinder,to accommodate the circulation of a coolant effective in maintaining acool running rotor at elevated current densities. Further in accordancewith the present invention, the cylindrical conductor is formed anddiffusion bonded to the rotor core with the coolant conduits totallyembedded in the conductor metal utilizing hot isostatic pressure (HIP).

The invention accordingly comprises the features of construction,conbination of elements, arrangement of parts, together with a method ofmanufacture, which will be exemplified in the construction and processsteps hereinafter set forth, and the scope of the invention will beindicated in the claims.

For a fuller understanding of the nature and objects of the invention,reference should be had to the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. 1 is a side view, partially in section, of an acyclic generatorrotor constructed in accordance with the invention;

FIG. 2 is a fragmentary perspective view of the rotor of FIG. 1 at anintermediary stage of its manufacture; and

FIG. 3 is an elevational view, partially broken away, schematicallyillustrating the formation of the rotor conductor utilizing hotisostatic pressure apparatus.

Like reference numerals refer to corresponding parts throughout theseveral views of the drawings.

DETAILED DESCRIPTION

Referring first to FIG. 1, an acyclic generator rotor, generallyindicated at 10, is comprised of a forged ferromagnetic steel core 12integrally formed with axial shaft extensions 14 and 16 by which therotor is journalled and driven to high peripheral velocities. Bonded tothe peripheral surface 12a of the core is a current conductor in theform of a cylinder 18 of a highly electrically conductive metal, such ascopper. This cylinder is appropriately machined to provide a pair ofaxially spaced, current collector ring surfaces 20 which cooperate withclosely spaced, conforming stator current collector ring surfaces and anintervening collector medium, such as liquid metal, to transport thehigh DC current developed in the rotor to the stator and ultimately thegenerator output terminals (not shown). Also machined into the cylinderperiphery in flanking relation to these rotor collector surfaces aresuitable shoulders, grooves and lands which cooperate with complementingstator surface formations in containing the liquid metal beingcirculated in the collector gaps. For a detailed description of anacyclic generator utilizing liquid metal collectors, reference may behad to U.S. Pat. No. 3,211,936, issued to L.M. Harvey and assigned tothe assignee of the instant application.

In accordance with the present invention, a concentric blind hole 22 isaxially bored through shaft extension 16 and into core 12, terminatingshort of shaft extension 14. This hole is counterbored, as indicated at24, to an axial depth beyond the right end of conductor cylinder 18. Astainless steel tube 26 is provided with a collar 28 which is weldedabout its inner end, such that the tube can be fitted into counter bore24 to the bottom thereof. To assure a reasonably fluid-tight fit, collar28 may be grooved to accommodate an O-ring seal 30. As will be seen,hole 22 together with tube 26 provide an axial passageway 32accommodating the incoming flow of a liquid coolant, while the annularspace between this tube and counterbore 24 provides a passageway 34accommodating return flow, as indicated by arrows 36. It will beappreciated that the exterior termination of tube 26 and counterbore 24are equipped with suitable rotating unions (not shown) forfluid-connecting these passageways with an appropriate heat exchangerand pump in closed loop fashion.

Still referring to FIG. 1, just short of the inner termination of hole22, a plurality of angularly spaced, radially directed passages 38 aredrilled in core 12 into communication with axial passageway 32.Similarly, a plurality of angularly spaced, radially directed passages40 are drilled in the core into communication with passageway 34 at apoint short of its inner termination at collar 28. Embedded in conductorcylinder 18 are a plurality of conduits, such as stainless steel tubes42, which extend substantially the entire axial length of the cylinder.One end of each tube is turned radially inwardly for insertion into theouter termination of a different one of the passages 38 and eitherwelded or brazed in place. Likewise the other ends of these tubes areturned radially inward, inserted in the outer ends of passages 40, andaffixed in place. It is thus seen that coolant can flow in throughpassageway 32, radially out through passages 38, axially through tubes42, radially inward through passages 40 and axially out throughpassageway 34. Obviously, the above described direction of coolantcirculation is arbitrary, and could be reversed. As seen in FIGS. 1 and2, passages 38 alternate between axially offset positions around thecircumference of core 12, as do passages 40, so as not to undulyprejudice the structural integrity of rotor 10. Also, as seen in FIG. 2,tubes 42 preferably serpentine axially back and forth an odd number oftimes, three in the illustration, in conveying coolant between passages38 and 40. Suitable coolants are DOWTHERM J, available from Dow ChemicalCompany and COOLANOL marketed by Monsanto Chemical Company.

In accordance with the present invention, conductor cylinder 18 isdiffusion bonded to the peripherial surface 12a of core 12 by a hotisostatic pressure (HIP) process and is at least in part created by thesame process. In one form of the process for manufacturing the rotor 10of FIG. 1, the peripherial surface 12a of the core is nickel plated, andthen a sheet 18a of, for example, oxygen dispersion strengthened (ODS)copper is cut to size and wrapped around the core periphery with itsaxially arranged, cut ends butted together along a seam 44, as seen inFIG. 2. The sheet is spot welded to the core at numerous points asindicated at 46 and then machined to provide a plurality of full-lengthaxially extending grooves 48 semi-circular in cross section such as toconform to the circular contour of tubes 42. The straight sections ofthese cooling tubes are seated in the grooves and held in place bysuitable means such as welding, brazing or circumferential copper bands(not shown), leaving the tubes end-turn sections overhanging the axialends of sheet 18a to facilitate their connections with radial passages38 and 40. The integrity of these connections is checked with a massspectrometer. Preferably, as in the case of the rotor, tubes 42 arenickel plated prior to installation, to promote diffusion bonding withcopper.

Turning to FIG. 3, a metallic container 50, formed of a mild steel or analloy such as inconel 600, is fabricated to envelope the cooling tubes42 as assembled to the core (FIG. 2) and is affixed in sealed relationto core surface 12a by continuous circumferential weldments 52. Thecontainer is then loaded with an appropriate metallic powder 54, such asMZC copper powder, comprising, for example, 0.05% magnesium, 0.195%zirconium, 0.35% chromium and the remainder copper, through a fill tube56. The assembly is vibrated during loading to compact the powder incontainer 50, and then heated in a furnace at approximately 1000° F.while maintaining a rough vacuum to outgas the powder 54. Fill tube 56is then sealed off as illustrated in FIG. 3.

The assembly is then placed in a hot isostatic press or autoclave 58which is raised to a temperature of approximately 1825° F. and apressure of approximately 15,000 psi. These temperature and pressureconditions are held for a suitable period, such as two hours. It will beappreciated that these process parameters depend on the character ofcopper powder loaded into container 50. The container is crushed underthis pressure, and its copper powder contents are densified to anon-porous mass which is diffusion bonded to the surfaces of core 12,sheet 18a and tubes 42 exposed within the container. Moreover, sheet 18ais at the same time diffusion bonded to core surface 12a at theirinterface. It is important to note that pasageways 32 and 34 are ventedto the autoclave ambient, and thus tubes 42 are pressurized to theautoclave pressure to prevent their collapse during the HIP process. Thecrushed container 50 is machined away, and the exposed copper cylinderis rough machined to a symetrical shape. The rotor assembly is then heattreated in an inert gas environment, such as argon, at, for example,1825° F. for one hour to relieve any stresses. Thereafter, the rotorassembly is rapidly cooled down to 850° F. and then aged in air for asuitable period, such as four hours to develop maximum tensileproperties in the core 12 and conductor 18. Final machining can then beundertaken to create the surface configuration of cylindrical conductor18 seen in FIG. 1 with cooling tubes 42 fully embedded therein anddiffusion bonded at their copper-steel interfaces.

As an alternative to copper sheet 18a, its equivalent can be created byan intermediary HIP process. Thus, core 12 is nickel plated, and acontainer, similar to but appropriately smaller than container 50, iswelded in place. This container is filled with MZC copper powder,vibrated to compact the fill, and heated under vacuum to outgas thepowder. The core and filled container are subjected to a HIP process tothus create a densified non-porous copper mass in the form of a sleeveor layer diffusion bonded to the core surface. This container is thenmachined away, and the sleeve is machined to a cylindrical shape and thearray of axial grooves are cut into its peripheral surface in the mannerof FIG. 2. The radial passages 38 and 40 are bored, and the nickelplated cooling tubes 42 installed in the manner discribed above tocreate the assembly seen in FIG. 2. The HIP process described above inconjunction with FIG. 3 is then performed. The assembly is roughmachined, heat treated and finally machined to achieve the rotor of FIG.1, all the fashion described above.

From the foregoing, it is seen that the present invention provides aninternally cooled rotor for an acyclic generator which is capable ofaccommodating an order of magnitude increase in power density and dutycycle length, while enabling a significant reduction in the intervalbetween duty cycles. Moreover, by virtue of the hot isostatic pressureprocess to effectively implant a multiplicity of cooling tubes with thebulk of the rotor conductor, a cool-running rotor is achieved at theseelevated power densities. Moreover, by virtue of the present invention,the presence of these embedded cooling tubes does not prejudice, but infact preserves the structural integrity of the rotor conductor at highperipheral velocities by limiting temperature rise in the rotorconductor. While the present invention has been described in itsapplication to acyclic generation, it will be appreciated that it isequally applicable to acyclic motors.

It is thus seen that the objects set forth above, including those madeapparent from the preceding description, are efficiently attained and,since certain changes may be made in the above construction or steps ofthe above method without departing from the scope of the invention, itis intended that all matters contained in the above description or shownin the accompanying drawings shall be interpreted as illustrative andnot in a limiting sense.

Having described the invention, what is claimed as new and desired tosecure by Letters Patent is:
 1. A method for manufacturing a fluidcooled rotor for an acyclic electromagnetic machine comprising the stepsof:A. providing a rotor core of a ferromagnetic steel having aperipheral surface; B. machining internal passages in said core openinginto said core peripheral surface for accommodating the circulation of acoolant; C. providing a plurality of coolant tubes; D. fixedlysupporting said coolant tubes in spaced relation to said peripheralsurface, said tubes being distributed about the periphery of said core;E. coupling inlet and outlet ends of said tubes in coolant flowconnection with said passages at said peripheral surface; and F. forminga current conductor enveloping said tubes and diffusion bonded to saidcore peripheral surface from a metallic powder by hot isostaticpressure, said passages being vented to the isostatic pressure toprevent collapse of said tubes.
 2. The method defined in claim 1,wherein said metallic powder consists predominately of copper.
 3. Themethod defined in claim 1, which further includes the step of sealinglyaffixing a container about the periphery of said core for containingsaid metallic powder during the application of said hot isostaticpressure.
 4. The method defined in claim 3, which further includes thesteps of removing said container and machining said formed currentconductor to a desired surface configuration.
 5. The method defined inclaim 1, wherein said coolant tube supporting step is achieved byproviding a layer of current conductor material about said coreperiphery and secured to said peripheral surface thereof, machiningelongated surface grooves in the periphery of said layer, seating saidtubes in said grooves, and affixing said tubes in place, whereby saidcurrent conductor is formed by said hot isostatic pressure as anintegral composite of said layer diffusion bonded to said coreperipheral surface and said metallic powder as a densified non-porousmass diffusion bonded to the peripheral surface of said layer andlimited portions of said core peripheral surface beyond said layer,whereby said coolant tubes are fully embedded in said current conductor.6. The method defined in claim 5, wherein said layer is provided as asheet wrapped about the core periphery.
 7. The method defined in claim5, wherein said layer is also created as a densified non-porous massfrom metallic powder by the application of hot isostatic pressure. 8.The method defined in claim 5, wherein said layer and said mass consistpredominately of copper.
 9. The method defined in claim 5, which furtherincludes the step of sealingly affixing a container about the peripheryof said core for containing said metallic powder during the applicationof said hot isostatic pressure.
 10. The method defined in claim 9, whichfurther includes the steps of removing said container, heat treating thecore-current conductor assembly, and machining said formed currentconductor to a desired surface configuration.
 11. The method defined inclaim 10, wherein said layer and said mass consist predominately ofcopper.
 12. The method defined in claim 11, which further includes thestep of nickel plating said core peripheral surface to promote diffusionbonding of said layer and said mass thereto.
 13. The method defined inclaim 12, wherein said hot isostatic pressure is applied at atemperature of approximately 1825° F. and a pressure of approximately15,000 psi for approximately two hours.
 14. The method defined in claim13, which further includes the step of heating said container under avacuum to outgas said metallic powder therein.